Java Introduction

1. Java Introduction 密碼學與應用 海洋大學資訊工程系 丁培毅

2. Download JDK http://www.oracle.com/technetwork/java/javase/downloads/index.html

3. Java Program Development IDEs • Eclipse, http://www.eclipse.org/downloads/ Eclipse IDE for Java Developers, 151 MB • Apache Ant, http://ant.apache.org/bindownload.cgi • Apache Maven, http://maven.apache.org/docs/3.1.1/release-notes.html • Or simply notepad++ and cmd.exe (javac and java) jdk7u40.bat or modify 系統內容/進階/環境變數 @echo off set path=%path%;c:\progra~1\java\jdk1.7.0_40\bin set classpath=.;c:\progra~1\Java\jdk1.7.0_40\src.zip @echo on

4. Java Programs • Programs are built from classes • Classes define data storages and behaviors of objects (class instances) • data structure of an object defined by data members • behavior of an object are defined by methods that manipulate the data members.

5. HelloWorld.java class HelloWorld { public static void main(String[] arg) { System.out.println("Hello World"); } } Compiling: javac HelloWorld.java Executing: java HelloWorld

6. Factorial.java • factorial(21) overflows a 64-bit integer (long) • java.math.BigInteger supports arbitrarily long integers(java.math.BigDecimal supports arbitrarily long real numbers) import java.math.BigInteger; class BigFactorial { public static void main(String[] args) { BigInteger fact = new BigInteger("1"); for (int i=1; i<60; i++) { fact = fact.multiply(BigInteger.valueOf(i)); System.out.println(fact); } } }

7. Comments • Three forms /* comment */ // comment to end of line /** documentation comment for javadoc tool */ • all characters in Java programs are unicode (16 bit)

8. Constants • examples static final int face = Suit.DIAMONDS; class Suit { static final int CLUBS = 1; static final int DIAMONDS = 2; static final int HEARTS = 3; static final int SPADES = 4; }

9. Control Flow & Operators • control statements mostly cloned from C++ • differences: • boolean type requires operands of &&, || etc. to be boolean • while(1) not OK…while(true) is fine • + is string concatenation • I/O is not like C++ • arrays are not like C++

10. Classes and Objects • Fields are data variables associated with a class or with instances of the class • primitive data (int, double, …) • object data ( BigInteger r ) • Methods provide execution behavior and operate on the data fields • Classes and Interfaces can be members of other Classes or Interfaces.

11. Interfaces • Interfaces define the methods that may be used but do not specify instance storage. interface ConstAccount { double getBalance(); } class Account implements ConstAccount { double d_ = 0.0; void setBalance(double d) {d_ = d;} double getBalance() { return d_;} }

12. Creating Objects • Objects are primitive or user defined. • Primitive objects are by value, user defined objects are references. • User defined Objects are created with constructors and the “new” keyword. • Point center = new Point(4.0,5.9);

13. Static fields • static fields in a class or interface belong to the class. If there are instances of a class with static data fields then all instances share the static data fields • static methods are invoked using the class name.

14. Garbage Collection • When there are no active references to an object storage is automatically reclaimed by a garbage collector thread running in the background.

15. Methods and Parameters • methods operate on the data fields of a class. • methods have zero or more parameters and may return values or be void. • a methods name, number of parameters and their type make up the signature of the method. • two methods may share a name if they have different signatures.

16. Invoking a method • a non-static method is invoked using an object of the class and the “dot” operator. • Point p = new Point(3.2,3.3);p.clear(); • the object on which the method is invoked is the “receiver”. The method is a “message” to the object.

17. “this” • the keyword “this” refers to the current receiving object. • public void clear() {this.x = 0.0; y = 0.0; // this is assumed }

18. Arrays • An array is a collection of variables of the same type • Card [] deck = new Card[52];for (int i = 0; i < deck.length; i++) { deck[i] = . . .

19. Strings • String objects are immutable objects. • String myName = “W. H. Carlisle”; • System.out.println( myName + “ III”); • Use stringBuffer objects if you wish to manipulate the storage. • there are methods that can be invoked on string objects. • myName.length()

20. Inheritance • Classes may extend other classes. • variables of a subclass with the same name as those of a superclass shadow the superclass storage. • a method of a subclass with the same signature as that of a superclass overrides the superclass method. • objects of a subclass may be used in superclass variables.

21. Inheritance • if a method is overridden, the keyword “super” refers to the superclass. • Classes may extend only one class. If a class does not extend a class by default it extends the class Object. • A class may implement many interfaces.

22. Exceptions • The exception mechanism of Java is very similar to that of C++ try { } catch( Exception e) { } • if a method throws a “checked” exception the compiler requires that a handler catch the exception or that the invoking method also indicates that it also throws the exception. • a “finally” block may follow “catches” to close files, etc.

23. Packages • packages are Java’s way to manage name spaces. • Packages are implemented as directories within a file system. When package names must span systems, the common practice is to reverse the internet domain name import COM.Sun.games; • import is not inclusion and is only for the compiler. Class paths are used to find the class information at run time.